Hormones maintain and regulate body systems and act on specific target cells, regulating physiological function. They are secreted in any of three patterns—diurnal, pulsatile, or cyclical—or in patterns that are dependent on the levels of circulating substrates, such as electrolytes or minerals. Hormones bind onto specific receptor sites, not on cells or tissue. Generally, hormones are metabolized in the liver and excreted by the kidneys. Hormone release can be regulated by chemical factors, endocrine or hormonal factors, and/or neural factors. Feedback systems function to keep hormone levels within physiological ranges. The complexity of the feedback systems of the endocrine glands contributes to the difficulty in timely diagnosis of endocrine disorders. Any dysfunction of a particular gland causes decreased release of hormones that may produce adverse effects on specific body systems.